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Do You Know the Answers to These Antennas Questions?




 
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K1JJ
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"Let's go sailing, Tommy!" - Yaz


« on: February 23, 2005, 01:25:14 PM »

Ola Guys,

Sit back and enjoy some interesting reading.......

Over the last six months I've been spending some time up on 6M and have been talking with and "developing" a group of enthusiastic hams.  Most are tech/general newbies who want to improve their stations and antenna systems.  They are all running simple 3 or 5 element Yagis, but now want to lengthen booms, stack antennas , etc.  Some are now talking about BUILDING linears rather than buying. Good crowd.

Well anyway, I've gotten axed some great questions lately about antennas that I realized I did not know the answers to off hand. I told them I would check out things on the modeling program and get back. Here's the results of a few emails and on-the-air discussions that I thought I'd share with you guys here. Some of the concepts are simple, but were not obvious, at least to me...

As we know, vertical take-off angle on an antenna is EVERYTHING! It's what makes a high Yagi pissweak locally and a low dipole loud. And what makes a high Yagi hear the DX while a low dipole hears static. On the low bands there may not be much we can do about it because of height limitations, but on the higher frequencies we can often tailor and optimize our antenna system heights to match whatever operations we prefer to do.


BTW, many AMers are already on 6M!  Considering putting up a Yagi and joining the fun. We all need a break away from HF once in a while!... :grin:

eMails as follows and applies to 6 Meters:
---------------------------

Hi Guys,

Ray asked a coupla of questions tonight that I couldn't answer, so I did some modeling to find the answers....

Ques 1:

Does the LOWEST take-off angle lobe of a Yagi at a given height get lower if the boom is lengthened?  ie, a 6M Yagi at 50' is on a 24' boom and then gets lengthened to 48'. (Still at 50' high and 2.5 wavelengths high)

The answer is NO, the lowest lobe does not get lower **IF**  the Yagi is at ROUGHLY 3/4 wavelength or higher above the ground..  The lowest main lobe does not get lower with a longer boom. It DOES get lower *IF* the Yagi is LESS than roughly 3/4 wavelengtn high. (less than 15' high on 6M)

This LOWER height "cross-over point" is variable and depends upon the boom length. ie, for a 3el Yagi the cross-over is at about 3/4 wavelengths high. For a 6el Yagi it is more around 1/2 wavelength above ground.   So let's say it varies from 1/2-3/4 wavelength for average sized beams. Consult your own modeling program when near this area.

When higher than 3/4 wavelength above ground, the higher lobes start to move down and form into the lowest lobe to make it fatter. But this lowest lobe stays the same take-off in degrees. The higher angle energy is forced towards the horizon all into the lower lobe as the boom lengthens greatly.  So, a 3el Yagi at 50' has MANY lobes with the lowest is at 6 degrees. A  200' long boom Yagi has only one big lobe at 6 degrees and a small minor one up at about 20 degrees.  The majority of the energy is in the lowest lobe with a very LONG Yagi that is within a wavelength of the ground.  The higher it goes up above the ground, the more minor lobes start coming back into the picture thru reflections.


Ques: 2)  Does the take-off angle halve if the height is doubled?

I  took a 24' boom Yagi and put it at heights from 3' to 200' high and
determined the take-off angles below in degrees.  You will see that it
starts out non-linear at the lowest heights, but seems to get linear once
1/2 - 1 wave off the ground. Check this out:

6M  24' boom Yagi at:

3' high  -   34 degree take-off angle
6'.......  25 degrees
12.5'......17 degrees
25'......12 degrees    (1.25 wavelength high)
50'......  6 degrees    (2.5 wavelengths high)
100'..... 3 degrees
200'...... 1.5 degrees

Ray....   your 24' boomed Yagi at 25' has a take-off angle of 12 degrees,
not 6 degrees. I was looking at the wrong plot when I told you. It is still
down about -7db at 3 degrees.


----------------------------------

So, to summerize  -  as mentioned above......

As it turns out, when lengthening a boom, it DOES make a lower take-off
angle, but ONLY when the Yagi is less than roughly 1/2- 3/4 wavelength above ground, depending upon starting boom length. The lower the Yagi is below 3/4 wavelength, the more dramatic the longer boom has on the lowest lobe coming down lower. This is very significant on HF freqs where 130' is  1/2 wavelength on 75M, etc.

However, on 6M, 3/4 wavelength is only about 15', and most modest ham antennas are above this height. So in this case a longer boom has little to no effect on lowering the LOWEST lobe. But it does bring down the higher lobes and mix them into the lowest, resulting in a fatter lowest lobe as the boom increases length.

Bottom Line: With Yagis > 3/4 wavelength high, the lowest lobe take-off angle is based almost soley on height above ground. It will be changed only in "thickness" by vertical stacking, and ground reflections.
----------------------------------

BTW, here's the goods - what each wavelength above ground produces for a takeoff angle - notice all but one are all above 3/4 wavelength high, so a longer boom will have little effect on the LOWEST lobe. This works for any frequency.  Shown for 6M in feet.  The degree readings are approximate as limited by the polar resolution on my computer screen:


48' boom 9 el Yagi Yagi

HEIGHT FT    WAVELENGTH         DEGREES

9.8'....   1/2 wavelength.......  17 degrees  (lowest lobe take-off angle)
19.6'.... 1 wavelength ........   12 degrees
39.2'.... 2 wavelengths........   7 degrees
58.8'.... 3  ..........................    5 degrees
78.4'.... 4 ...........................     4 degrees
98'.........5..........................      3 degrees
117'...... 6............................... 2 degrees
137'.....  7................................ 1.8? degrees
156' ..... 8................................. 1.6? degrees


6' boom 3el Yagi

HEIGHT FT    WAVELENGTH         DEGREES

9.8'....   1/2 wavelength.......  28 degrees  (lowest lobe take-off angle)
19.6'.... 1 wavelength ........   15 degrees
39.2'.... 2 wavelengths........   7 degrees


Notice the huge difference boom length makes at 1/2 wavelength high comparing a 6' to a 48' boom. (17 vs: 28 degrees) However, the take-off angle gets real close when they are both at 2 wavelengths above ground.
Bear in mind this is an extreme example comparing a 6' to 48' boom, but you get the idea...  the cross-over point is not digital, it's a a little rough...
-----------------------------

HORIZONTAL vs: VERTICAL STACKING of YAGIs -  (side by side vs: one over the other)


Question:  You have two Yagis. Is it better to vertically or horizonally stack them?

Like everything, it all depends upon what you want to do with them. DX only, local ground wave,  E skip, combination of all ... , etc.


Here's a comparsion I did of a pair of 48' boom, 9el Yagis on 6M.

In the first listing is the pair vertically stacked at 100' and 64'.  (36'
spacing)  They are shown in-phase and 180 degrees out-of-phase.

The second is with the two Yagis horizontally stacked, 36' apart at the
booms.  Shown only in-phase.

I show the major lobe amplitudes in db at the various angles. You will see
that EVEN when the vertical stack is switched 0/0  or 0/180 to get more high angles, the horiz stack is always stronger in amplitude, though has thinner lobes at the horizon. (3 degrees)  The same relative differences will apply to your 24' pair, Ray. Very interesting.  Print it out to be able to look at all three colums to see the big picture.  Take each angle at a time and compare the gain to the others.

This is OVER GROUND, so there is ground reflection gain...  just look at the
differences between the three to get the idea.

***** Vertical take-off in degrees vs: gain:  (Fed in phase) *******

2 Yagis Collinear (Horiz stacked):

3 degrees....23dbi
9.............. 22 dbi
15............ 21 dbi
20............ 19 dbi
27............ 15 dbi

---------------------------------------

Vertical Stacked  - both in phase:

3 degrees......... 22.57 dbi
10.................... 16.5   dbi
20................... 6.5  dbi
25................... 14.5 dbi


Vertical stacked - out of phase:
8 degrees ............... 18.8 dbi
13........................... 20.8
20..........................  16.8

The main difference here and not shown without the polar plot is the fact that the vertically stacked lobe at 3, 8 and 13  degrees are MUCH fatter (and fewer) than the collinear horiz stacked Yagis. This is an advantage assuming you are interested in putting the majority of your power near the horizon at the expense of the other higher angles.  Thats why vert stacking is good for F2 propagation, etc.  But with the MANY incoming angles we deal with on 6M it's better to have thinner lobes that are more distributed as shown with higher gain.  At least that's what I plan to do with my new experimental 4 X 48'  boom Yagis in a horizontal line - 98' cross boom - yikes!

------------------------


Question:  How would you modify an M2 5 el 18' boom Yagi for more gain?

I would lengthen the boom to 24' and retune the elements and spacing to gain an extra 1.5 db. A 6M 5el Yagi "wants" to be on a longer 24' boom if you want max forward gain with a compromised 22db f-b ratio and 12.6 ohms for a feedpoint to match the 4:1 match that M2 uses now.


Here's the dimensions to build from scratch or modify an existing 5el Yagi. (Gary/ INR, here's the antenna we talked about for you to build)

5 el 6M Yagi  (designed for 1/2" aluminum tubing elements and a single 2" diameter .125" thick wall,  24' boom.)


Gain: 12.46 dbi (free space)
Two horiz stacked at 29.4'  spacing:  15.58dbi

F-B:  22.16 db

Position on boom Inches....Length (1/2)  (half lengths)
Ref: 0".............................57.480"  X2
Driven: 43.152".................55.000
D1:  106.245.....................53.206
D2:  194.222.....................52.183
D3:   270.832.....................52.150

This is optimized for a compromise 22db f-b  so that the boom is now 22'  7" long. This allows room for the end element boom to element plates.

Input impedance is 12.6 ohms +J2

If used with the boom above, "YagiStress" mechanical modeling program  says no overhead trussing support is needed. I have one up now and it has survived ice storms and wind with no overhead supports.

The overall design looks very good.


73,
Tom, K1JJ
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #1 on: February 23, 2005, 02:11:56 PM »

Some very good information here Tom. Should help a lot with antenna related questions, i.e. big, small, number of elements, stacking, etc. for someone itching to get on 6 meters. Now, if we could only predict the time when the band will open to certain areas without having to sit in front of the rig 24/7, we would have it made. The sun, weather, earth rotation, meteors, D, E, F layers, etc. all play a part in 6 meter excitement.
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« Reply #2 on: February 23, 2005, 03:31:19 PM »

Tom Uv expands to VHF

But why was I able to work KH6 on 20M with 50 watts when my log was sitting on the kid's swing set at 6 feet off the ground? easy 2 hops.
The crazy results like that make you think.  fc
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K1JJ
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« Reply #3 on: February 23, 2005, 03:44:37 PM »

Quote from: WA1GFZ
But why was I able to work KH6 on 20M with 50 watts when my log was sitting on the kid's swing set at 6 feet off the ground? easy 2 hops. The crazy results like that make you think.  fc


It's high angle, baby....  ahhhh, love those log periodics that will play laying on the ground if need be!  And then again, maybe you would have gained another 20db if the log was raised to 60' like it is now...

On 6M, sometimes the optimum angle comes in so high that my Yagi shows little pattern wherever it's rotated. That's how you know.  (Look at the pattern of most horizontal antennas from a bird's eye view and notice they are almost circular.)

What shocked me the most about 6M is when HIGH angle E skip is coming in, the guys with Yagis on the house roof at 20' are working the same very PW guys out west that I am with my high stacks. In fact, they had the advantage when angles get as high as 30-60 degrees...  :lol:

But when the angles start coming in and hugging the horizon at 1-2 degrees like a 350-400 mile over-the-horizon ground path or marginal F2 skip into eastern Eu, etc, the low Yagis are deaf.


Another weird example is VK6LK from Australia on 3798 every night. He comes in VERY high angle most of the time. Guys with dipoles at 30' work him. Whereas, at the same time Kan/7J4AAL in Hiroshima comes in at angles so low that only a handful of guys with very low angle antennas can even hear him most of the time, never mind work him. Go figure.

I guess, bottom line is that we need BOTH antenna types to handle it all, caw mawn.

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
Steve - WB3HUZ
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« Reply #4 on: February 24, 2005, 12:30:34 PM »

How high should my MoonRaker be to plug all the mud ducks' ears on The Double Deuce?
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N3DRB The Derb
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« Reply #5 on: February 24, 2005, 05:46:29 PM »

Quote
How high should my MoonRaker be to plug all the mud ducks' ears on The Double Deuce?


Thats the only thing that interests me as well.  :cool:
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K1KV
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« Reply #6 on: February 25, 2005, 07:45:33 AM »

'bout as high as you can throw an empty Billy-beer can.
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